1. Field of the Invention
This invention relates generally to a knee joint prosthesis which replaces the articulating knee portion of the femur and tibia, and more particularly, to a floating bearing knee joint prosthesis having a fixed tibial post.
2. Discussion of the Related Art
A knee joint prosthesis typically comprises a femoral component and a tibial component. The femoral component and the tibial component are designed to be surgically attached to the distal end of the femur and the proximal end of the tibia, respectively. The femoral component is further designed to cooperate with the tibial component in simulating and articulating motion of an anatomical knee joint.
Motion of a natural knee is kinematically complex. During a relatively broad range of flexion and extension, the articular or bearing surfaces of a natural knee experience rotation, medial and lateral angulation, translation in the sagittal plane, rollback and sliding. Knee joint prostheses, in combination with ligaments and muscles, attempt to duplicate this natural knee motion, as well as absorb and control forces generated during the range of flexion. Depending on the degree of damage or deterioration of the knee tendons and ligaments, however, it may be necessary for a knee joint prosthesis to eliminate one or more of these motions in order to provide adequate stability.
While knee joint prostheses are effective in replacing the anatomical knee joint, they nevertheless follow different design options that each have different advantages and disadvantages. For example, many knee joint prostheses are polycentric, such that the posterior femoral radius is smaller than the distal femoral radius. The reason for the change in radius is to provide an appropriately sized femoral component, facilitate early rollback, and to allow for full flexion. However, because of the smaller posterior femoral radius, such knee joint prostheses provide a large articulating contact area in extension, but as the knee joint prosthesis moves during flexion, the smaller posterior radius creates a smaller articulating contact area as flexion increases. This generally reduces the contact area from a large surface contact area to a smaller contact area between the femoral component and the tibial component. For example, the contact area may be about 275 mm2 at extension and reduce substantially to about 75 mm2 at 60xc2x0 to 90xc2x0 of flexion. This reduced contact area may not be optimal for long term durability due to increased contact stresses.
Other knee joint prostheses attempt to eliminate this disadvantage by using a roller and trough articulation, where the roller and trough are spherically or cylinderally shaped to provide full contact area throughout the range of motion from extension to flexion. However, the disadvantage with most of these type knee joint prostheses is that posterior rollback of the femoral component relative to the tibial component is eliminated. Such rollback is extremely desirable because it increases the extension moment arm, increases quadriceps efficiency, and helps patients perform activities of daily living, such as climbing stairs. In addition, these types of designs are typically rotationally constrained.
Other knee joint prostheses attempt to eliminate the above disadvantages using another design option, such as a meniscus or floating bearing between the femoral component and the tibial component. By utilizing a floating bearing, the articulating contact area between the femoral component and the bearing can theoretically be increased without increasing constraint. However, some of the knee joint prostheses that provide a floating bearing rely on ligaments for femoral rollback which in many cases may not be reliable. Moreover, some floating bearings may dislocate due to the shape of the bearing itself and the moment arms generated during rollback.
Knee joint prostheses are also generally provided having different levels of constraint. For example, primary type knee joint prostheses provide the least level of constraint, posterior stabilized (PS) knee joint prostheses provide an intermediate level of constraint, while fully constrained type knee joint prostheses provide the highest level of constraint upon the kinematic motions of a knee joint. In some situations, a patient may initially have a less constrained type knee joint implanted, such as a primary or posterior stabilized (PS) knee joint prosthesis. Should the patient later require a fully constrained type knee joint prosthesis because the patient is exhibiting instability, a surgeon is generally required to remove the entire knee joint prosthesis and implant both a new femoral and tibial component or a new fixed bearing to provide a fully constrained knee joint prosthesis. However, such extensive surgical modifications increases the overall surgical cost and complexity of upgrading a knee joint prosthesis from one constraint to another.
What is needed then is a knee joint prosthesis which does not suffer from the above mentioned disadvantages. This in turn, will provide a substantially conforming contact area between the articulating surfaces of the femoral component and the tibial component after extension, provide an increasing articulating contact surface area from extension through flexion, reduce overall stresses and wear in the articulating contact area, provide femoral rollback relative to the tibial component by means of a floating bearing, provide a floating bearing that reduces or eliminates bearing dislocation, provide a floating bearing which may reduce soft tissue impingement in extension, provide a mechanical engagement mechanism to force femoral rollback without having to rely on soft tissue ligaments, increase the overall reliability of the knee joint prosthesis, prevent anterior movement of the bearing and provide a modular knee joint prosthesis that can achieve different levels of constraint by simply replacing a guide post that is fixed to the tibial component. It is, therefore, an object of the present invention to provide a floating or rotating bearing knee joint prosthesis with a fixed tibial post that achieves the above-identified advantages.
In accordance with the teachings of the present invention, a floating or rotating bearing knee joint prosthesis with a fixed tibial post for use in replacing the articulating knee portion of a femur and a tibia is disclosed. The floating bearing knee joint prosthesis with the fixed tibial post provides a contact surface that increases between a femoral component, a tibial component and a bearing member as flexion increases.
In one preferred embodiment, a knee joint prosthesis for replacing an articulating portion of a femur and a tibia includes a femoral component, a tibial component, a guide post and a bearing member. The femoral component includes an engagement member, a first femoral bearing surface and a second femoral bearing surface. The tibial component includes a tibial bearing surface. The guide post extends from the tibial component and is operable to be engaged by the engagement member of the femoral component. The bearing member includes a first bearing surface that articulates with the first femoral bearing surface, a second bearing surface that articulates with the second femoral bearing surface and a third bearing surface that articulates with the tibial bearing surface. The bearing member also includes a posterior lip extension extending posteriorly from said third bearing surface, such that the bearing member is substantially inhibited from dislocating upon flexion of the knee joint prosthesis.
In another preferred embodiment, the knee joint prosthesis includes a femoral component, a tibial component, a guide post and a bearing member. The femoral component includes an engagement member, a first femoral bearing surface and a second femoral bearing surface. The tibial component includes a tibial bearing surface. The guide post extends from the tibial component and is operable to be engaged by the engagement member of the femoral component. The bearing member includes a first bearing surface that articulates with the first femoral bearing surface, a second bearing surface that articulates with the second femoral bearing surface and a third bearing surface that articulates with the tibial bearing surface. The bearing member also defines an opening passing through the bearing member which receives the guide post through the opening in the bearing member. The guide post and opening are configured to substantially inhibit rotational movement of the bearing member relative to the tibial component in extension while providing greater rotational freedom of the bearing member relative to the tibial component as flexion of the knee joint prosthesis increases.
In yet another preferred embodiment, the knee joint prosthesis includes a femoral component, a tibial component, a guide post and a bearing member. The femoral component includes an engagement member, a first femoral bearing surface and a second femoral bearing surface. The tibial component includes a tibial bearing surface. The guide post extends from the tibial component and is operable to be engaged by the engagement member of the femoral component. The guide post also includes a pair of anterior lobes extending from the guide post. The bearing member includes a first bearing surface that articulates with the first femoral bearing surface, a second bearing surface that articulates with the second femoral bearing surface and a third bearing surface that articulates with the tibial bearing surface. The bearing member also defines an opening passing through the bearing member, such that the pair of anterior lobes are operable to engage a sidewall that defines the opening to control rotational movement of the bearing member relative to the tibial component.
The use of the present invention provides a floating bearing knee joint prosthesis with a fixed tibial post. The floating bearing knee joint prosthesis provides a contact surface that increases between its components as flexion increases. As a result, the aforementioned disadvantages associated with the currently available knee joint prostheses have been substantially reduced or eliminated.